Alcohol-related car crashes kill about 10,000 people each year in the United States and law enforcement wants more flexibility to determine whether a suspected drunk driver is, indeed, over the limit.

But the Supreme Court was clearly divided in oral arguments on Wednesday about whether police can obtain a blood test without a warrant to determine intoxication levels.

"One of the things that I think affects the view in this case is: It's a pretty scary image of somebody restrained, and a representative of the state approaching them with a needle," Chief Justice John Roberts said.

The justices as a group seemed reluctant to issue a bright line rule for either side and struggled to get a clear answer about how much time can elapse before police would reasonably be able to forgo warrants and order blood tests.

Half of U.S. states prohibit warrantless blood tests in "run-of-the-mill" drunk driving cases.

The appeal involves the case of Tyler McNeely, 25, who was stopped in the middle of the night two years ago for speeding near Cape Girardeau, Missouri.

He failed four field sobriety tests and then refused to take a portable Breathalyzer at the scene, records show. McNeely said he would not consent to a breath test in jail and was taken to a hospital.

A blood test was ordered unilaterally by the officer over McNeely's objections and the blood-alcohol reading was nearly twice the state's legal limit.

State courts subsequently divided over whether the test could be admitted as evidence, prompting the Supreme Court appeal.

Time is key

Law enforcement wants flexibility to conduct such "searches" without warrants, saying alcohol dissipates over time and that delays getting a judge to sign off on a blood sample can mean justice denied.

John Koester, Jr., an assistant prosecuting attorney, said during the arguments that the facts of the situation favor exceptions to the warrant requirement, essentially giving the officer in the field the benefit of the doubt in most cases.

The officer who arrested McNeely worried that it would take a "considerable amount of time" to secure a warrant, perhaps 90 minutes to two hours, in that rural area of the state.

Court records show the time lapse between the initial stop and the blood test was about 30 minutes.

The Fourth Amendment protects against unreasonable search and seizure and permits warrants only in cases of probable cause.

Justice Sonia Sotomayor raised that issue with the state's lawyer.

"How can it be reasonable to forgo the Fourth Amendment in a procedure as intrusive as a needle going into someone's body?" she asked. "The ruling by us today is going to change that and is going to -- if in your favor -- change that and put the court's imprint, on: Use the most intrusive way you [police] can to prove your case."

But Justice Antonin Scalia said there was usually only one practical reason for suspects to insist on a warrant.

"What do you think your client would really get from the warrant requirement other than the delay that would entail allowing his blood alcohol to reduce itself?" he asked McNeely's lawyer. "I don't see how the independent magistrate [signing the warrant] is going to do you a whole lot of good, except for the fact that it will delay the process."

The justices have said the warrant requirement can be suspended under extraordinary circumstances, such as the risk of endangering lives or destruction of evidence. But the court has also said state intrusions into one's own body generally require prior review and approval by a judge.

"There was nothing that distinguished this case on the facts from other cases on the facts" where a warrant was legally obtained," Justice Ruth Bader Ginsburg said.

U.S. supports state

The Obama administration is backing Missouri, saying there is a strong federal interest in deterring drunk driving.

A lawyer for the Justice Department said blood samples are often more accurate than breath tests.

Justice Samuel Alito, a former U.S. attorney, said his experience was that many magistrates were "unreceptive to receiving warrant applications in the middle of the night" making it impractical in many jurisdictions to secure a timely order.

Steven Shapiro, the legal director of the American Civil Liberties Union who represented McNeely, said there was "no doubt" that using a needle triggers a warrant requirement.

Neither states nor criminal suspects in general should expect a clear ruling in their favor. It is more likely the court will issue a nuanced opinion.

A victory allowing automatic blood draws would not stop other states from maintaining strict standards for their use. Indeed, the simple threat of a needle may be incentive enough for suspected drunk drivers to agree to less invasive breath tests.

Both sides concede that dynamic already exists in many cases.

But if McNeely prevails, police may be required to adopt more extensive guidelines on testing policies, narrowing the emergency exceptions to the warrant requirement.

The practical effect may be to force police to streamline and speed up the warrant process, so that drunk drivers are tested before it is too late to preserve the evidence.

The case is Missouri v. McNeely (11-1425). The ruling is due by the spring.